If your "new" sending unit is an aftermarket type offered by one of the parts vendors it will not work correctly. EVER. They are just not the right type of sender, not compatible with the stock gauge. I also bought one and it is useless. For the stock gauge you must use a stock sender. If you change out your fuel gauge that sender may work.

There was a long discussion about this issue over on the Fordbarn forum......unfortunately I found out AFTER buying it and trying to make it work. I found an original sender (put on a new float) and my gauge works perfectly.